Exhaust gas-purifying catalyst

ABSTRACT

An exhaust gas-purifying catalyst includes a support and a catalytic metal as one or more precious metals supported by the support. The support includes a composite oxide having a composition represented by a general formula AB α C β O 3 , wherein A represents one or more elements selected from the group consisting of lanthanum, neodymium, and yttrium, B represents iron or a combination of iron and aluminum, C represents one or more elements selected from the group consisting of iridium, ruthenium, tantalum, niobium, molybdenum, and tungsten, α and β each represents a numerical value within a range of more than 0 and less than 1, and α and β satisfy relational formulae of β&gt;α and α+β≦1.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Applications No. 2016-009920, filed Jan. 21, 2016, theentire contents of which are incorporated herein by reference.

FIELD

The present invention relates to an exhaust gas-purifying catalyst.

BACKGROUND

Conventionally, as an exhaust gas-purifying catalyst that treats anexhaust gas of an automobile, a three-way catalyst with a precious metalsuch as platinum supported by an inorganic oxide such as ceria oralumina has been widely used. In the three-way catalyst, the preciousmetal plays the role in promoting the reduction of nitrogen oxides andthe oxidations of carbon monoxide and hydrocarbons. The inorganic oxideplays the roles in increasing the specific surface area of the preciousmetal and suppressing the sintering of the precious metal by dissipatingheat generated by the reactions.

In recent years, occasions when an automotive vehicle such as anautomobile is driven at high-speed increase as the performance of anengine increases. Additionally, in order to prevent the pollution of theair, the regulations on the exhaust gas are made more stringent. Againstthese backdrops, temperature of the exhaust gas emitted by theautomotive vehicle is on the trend of rising. For that, in order toachieve the exhaust gas-purifying catalyst that exhibits sufficientperformance even when being used under such a condition, research anddevelopment are actively carried out.

For example, Jpn. Pat. Appln. KOKAI Publication No. 1-242149 describesNiAl₂O₄ generated during the reaction of alumina supporting a preciousmetal with nickel at high temperatures when nickel is used in order toremove hydrogen sulfide from an exhaust gas. The NiAl₂O₄ has a spinelstructure. The Patent Literature describes large deterioration in theactivity of a catalyst during the reaction. Furthermore, the PatentLiterature describes the effective use of a composite oxide containingceria and zirconia for the suppression of the reaction.

Jpn. Pat. Appln. KOKOKU Publication No. 6-75675 describes the graingrowth of ceria when being used at high temperatures, which causesdeterioration in oxygen storage capacity. The Patent Literaturedescribes the grain growth of a composite oxide represented by a generalformula Ce_(1-x)La_(x)O_(2-x/2) when being used at high temperatures,which causes deterioration in purification performance. The compositeoxide has a fluorite structure. Furthermore, in the Patent Literature,the following are described. Even when a composite oxide or a solidsolution that contains zirconia and ceria, wherein the atomic ratio ofzirconium and cerium is 5/95 to 70/30, is used at high temperatures, thegrain growth of the composite oxide or the solid solution is less likelyto occur.

Jpn. Pat. Appln. KOKAI Publication No. 10-202101 describes a supportcontaining alumina, ceria, and zirconia uniformly dispersed and havinghigh heat resistance.

Jpn. Pat. Appln. KOKAI Publication No. 2004-41866 describes a compositeoxide having a perovskite structure represented by a general formulaABPdO₃. In the general formula, the element A is at least one rare-earthelement such as La, Nd, and Y that exhibits a valence of 3 and cannotexhibit other valences. The element B is at least one element selectedfrom the group consisting of Al and transition elements other than Co,Pd, and rare-earth elements. The Patent Literature describes thecatalyst activity of palladium maintained at a high level for a longperiod of time when the composite oxide is used.

Jpn. Pat. Appln. KOKAI Publication No. 2004-41867 describes a compositeoxide having a perovskite structure represented by a general formulaABRhO₃. In the general formula, the element A is at least one rare-earthelement such as La, Nd, and Y that exhibits a valence of 3 and cannotexhibit other valences, or a combination of such a rare-earth elementand at least one of Ce and Pr. The element B is at least one elementselected from the group consisting of Al and transition elements otherthan Co, Rh, and rare-earth elements. The Patent Literature describesthe catalyst activity of rhodium maintained at a high level for a longperiod of time when the composite oxide is used. Jpn. Pat. Appln. KOKAIPublication No. 2004-41868 describes a composite oxide having aperovskite structure represented by a general formulaA_(1-x)A′_(x)B_(1-y-z)B′_(y)Pt_(z)O₃. In the general formula, theelement A is at least one rare-earth element such as La, Nd, and Y thatexhibits a valence of 3, and cannot exhibit other valences. The elementA′ is at least one element selected from an alkaline-earth metal and Ag.The element B is at least one element selected from Fe, Mn, and Al. Theelement B′ is at least one element selected from transition elementsother than Pt, Fe, Mn, Co, and rare-earth elements. The PatentLiterature describes the catalyst activity of platinum maintained at ahigh level for a long period of time when the composite oxide is used.

SUMMARY

An exhaust gas-purifying catalyst containing a precious metal such aspalladium is apt to cause the sintering of the precious metal when afuel-rich high temperature exhaust gas is supplied.

Thus, an object of the present invention is to provide an exhaustgas-purifying catalyst that is less prone to cause deterioration inperformance due to the sintering of a precious metal.

According to an aspect of the present invention, there is provided anexhaust gas-purifying catalyst comprising a support including acomposite oxide having a composition represented by a general formulaAB_(α)C_(β)O₃, wherein A represents one or more elements selected fromthe group consisting of lanthanum, neodymium, and yttrium, B representsiron or a combination of iron and aluminum, C represents one or moreelements selected from the group consisting of iridium, ruthenium,tantalum, niobium, molybdenum, and tungsten, α and β each represents anumerical value within a range of more than 0 and less than 1, and α andβ satisfy relational formulae of β<α and α+β≦1; and a catalytic metal asone or more precious metals supported by the support. Examples of thecomposite oxide having the composition represented by the generalformula include a composite oxide having oxygen vacancies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing an exhaustgas-purifying catalyst according to an embodiment of the presentinvention;

FIG. 2 is an enlarged cross-sectional view showing a part of the exhaustgas-purifying catalyst shown in FIG. 1;

FIG. 3 is a cross-sectional view schematically showing a state of acatalyst layer of the exhaust gas-purifying catalyst shown in FIG. 1 ina fuel-lean high temperature atmosphere;

FIG. 4 is a cross-sectional view schematically showing a state of acatalyst layer of the exhaust gas-purifying catalyst shown in FIG. 1 ina fuel-rich high temperature atmosphere;

FIG. 5 is a graph showing an example of influence that the compositionof a support exerts on the performance of the exhaust gas-purifyingcatalyst;

FIG. 6 is a graph showing another example of influence that thecomposition of a support exerts on the performance of the exhaustgas-purifying catalyst;

FIG. 7 is an electron microscope photograph of a catalyst layer obtainedfor a catalyst C1 after an endurance test; and

FIG. 8 is an electron microscope photograph of a catalyst layer obtainedfor a catalyst C24 after an endurance test.

DETAILED DESCRIPTION

Embodiments of the present invention will be described in detail belowwith reference to the drawings. In all the drawings, elements that arethe same or similar in function are given the same reference characters,and their description will not be repeated.

FIG. 1 is a perspective view schematically showing an exhaustgas-purifying catalyst according to an aspect of the present invention.FIG. 2 is an enlarged cross-sectional view showing a part of the exhaustgas-purifying catalyst shown in FIG. 1. FIG. 3 is a cross-sectional viewschematically showing a state of a catalyst layer of the exhaustgas-purifying catalyst shown in FIG. 1 in a fuel-lean high temperatureatmosphere. FIG. 4 is a cross-sectional view schematically showing astate of a catalyst layer of the exhaust gas-purifying catalyst shown inFIG. 1 in a fuel-rich high temperature atmosphere.

An exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 is a monolithcatalyst. The exhaust gas-purifying catalyst 1 contains a substrate 2such as a monolith honeycomb substrate. Typically, the substrate 2 ismade of ceramic such as cordierite.

A catalyst layer 3 is formed on a partition wall of the substrate 2. Thecatalyst layer 3 contains a support 31 and a catalytic metal 32 that areshown in FIGS. 3 and 4.

The support 31 exists in the form of particles, for example. In thiscase, the average particle diameter of the support 31 is, for example,within a range of 0.5 to 100 μm, and typically within a range of 1 to 20μm. The “average particle diameter” means a value obtained by thefollowing method.

First, a part of the catalyst layer 3 is removed from the exhaustgas-purifying catalyst 1. Next, using a scanning electron microscope(SEM), an SEM image of this sample is taken at a 2,500 to 50,000-foldmagnification. Twenty-five SEM images are taken. Then, the fourparticles in full view are randomly selected from the supports 31 in theSEM images, and the area is obtained for each of the selected particles.Diameters of circles having the same areas as the above-described areasare calculated, and the arithmetic mean of the diameters is obtained.The arithmetic mean is stated as the average particle diameter. Thestandard deviation of the average particle diameter is 15 μm or less.

The support 31 plays the roles in increasing the specific surface areaof the catalytic metal 32 and suppressing the sintering of the catalyticmetal 32 by dissipating heat generated by the reactions. Another rolesof the support 31 will be described in detail later.

The support 31 includes a composite oxide having a compositionrepresented by the following general formula:

AB_(α)C_(β)O₃

The composite oxide is a compound represented by a general formulaAB_(α)C_(β)O₃, for example. Alternatively, the composite oxide is amixture containing a plurality of compounds each represented by ageneral formula AB_(α)C_(β)O₃.

In the general formula, A is one or more elements selected from thegroup consisting of one or more rare earth elements whose valence doesnot deviate from trivalent, i.e., lanthanum, neodymium, and yttrium.Typically, the element A is lanthanum or a combination of lanthanum andneodymium or yttrium. In the latter case, the ratio of lanthanum in theelement A is, for example, 60 atomic % or more, and typically 70 atomic% or more. Also, in this case, the ratio of lanthanum in the element Ais, for example, 99 atomic % or less, and typically 95 atomic % or less.

The element B is iron or a combination of iron and aluminum. In thelatter case, the ratio of iron in the element B is, for example, 50atomic % or more, and typically 60 atomic % or more. Also, in this case,the ratio of iron in the element B is, for example, 99 atomic % or less,and typically 95 atomic % or less.

C represents one or more elements selected from the group consisting ofiridium, ruthenium, tantalum, niobium, molybdenum, and tungsten. Moretypically, the element C is iridium or a combination of iridium andother element. In the latter case, the ratio of iridium in the element Cis, for example, 35 atomic % or more, and typically 50 atomic % or more.Also, in this case, the ratio of iridium in the element C is, forexample, 99 atomic % or less, and typically 95 atomic % or less.

α and β each represents a numerical value within a range of more than 0and less than 1, and satisfy relational formulae of β<α and α+β≦1. Forexample, α and β satisfy relational formulae of β<α and 0.901≦α+β≦1.

β can be specified according to the following formula using α.

$\beta = {\left( {1 - \alpha} \right) \times \frac{{solid}\mspace{14mu} {solution}\text{-}{forming}\mspace{14mu} {ratio}\mspace{14mu} (\%)\mspace{14mu} {of}\mspace{14mu} {element}\mspace{14mu} C}{100}}$

Here, the solid solution-forming ratio (%) of the element C means avalue obtained as follows. First, an exhaust gas-purifying catalyst isheated to high temperatures in an oxidizing atmosphere, for example,heated at 1000° C. in an air atmosphere for 1 hour. A part of the heatedexhaust gas-purifying catalyst is taken, and immersed for 12 hours in25% hydrochloric acid at room temperature, which allows only thecomposite oxide to be dissolved in the hydrochloric acid. Thehydrochloric acid in which the composite oxide is dissolved is filteredto produce a filtrate, and the filtrate is subjected to inductivelycoupled plasma (ICP) spectrometry to obtain the content of the element Cin the filtrate. From the content of the element C in the filtrate, theproportion of the element C solid-solutioned in the composite oxide,i.e., the solid solution-forming ratio of the element C is obtained.

α is, for example, within a range of 0.7 to 0.9995, and β is, forexample, within a range of 0.000475 to 0.201. α is typically within arange of 0.9 to 0.999, and β is typically within a range of 0.00094 to0.083.

When the composite oxide is a compound having the compositionrepresented by the general formula, α and β satisfy the aboveconditions. When the composite oxide is a mixture containing a pluralityof compounds each represented by the general formula, α and β satisfythe above conditions, for example, in each of the compounds contained inthe composite oxide. When β is increased, the effect of suppressing thesintering of the catalytic metal 32 is enhanced. It is, however,difficult to manufacture a composite oxide having increased β.

The compound represented by the general formula is a compound having aperovskite structure, for example.

The compound represented by the general formula may have oxygenvacancies. Similarly, examples of the composite oxide described hereininclude a compound having a smaller oxygen molar fraction in addition tothe compounds represented by the general formula.

Since the compound represented by the general formula contains iron thatcan change its valence as the element B, the element C is likely to besolid-solutioned in a support 31 when an oxygen concentration in theatmosphere is high under a high temperature condition. When the oxygenconcentration in the atmosphere is low under the high temperaturecondition, the element C precipitates out of the support 31 and forms analloy with the catalytic metal 32 at high efficiency. The element Cproduces fine particles whenever it precipitates out of the support 31.The melting point of the catalytic metal 32 is increased when it isalloyed with the element C. Furthermore, when the oxygen concentrationin the atmosphere is high under the high temperature condition, theelement C is less prone to cause evaporation because the element C issolid-solutioned in the support 31. Therefore, the exhaust gas-purifyingcatalyst 1 is less likely to cause the sintering of the catalytic metal32, which maintains the performance over a long period of time.

On the other hand, when the compound represented by the general formulaAB_(α)C_(β)O₃ contains an element causing no valence change as theelement B, for example, in the case of a composite oxideMgTi_(0.99)Ru_(0.0091)O₃, the distortion of the crystal structure of thecomposite oxide is limited because the element B (titanium) causing novalence change. Thus, the solid-solution formation of the element C(ruthenium) with the support and the precipitation of the element C outof the support are less prone to occur. Therefore, such an exhaustgas-purifying catalyst is less likely to cause alloy formation betweenthe element C (ruthenium) and the catalytic metal on the support, andthus the sintering of the catalytic metal on the support is prone tooccur.

Typically, the support 31 shown in FIGS. 3 and 4 further contains theelement C, for example, the same element C as that contained in thecomposite oxide in the form of an elemental metal that is notsolid-solutioned in the composite oxide in addition to theabove-mentioned composite oxide. The ratio of the element C in the formof the elemental metal to the total amount of the element C contained inthe exhaust gas-purifying catalyst 1 is, for example, 80 atomic % orless, and typically 65 atomic % or less. The ratio is, for example, 1atomic % or more, and typically 5 atomic % or more.

The ratio of the amount of the element C contained in the compositeoxide to the total amount of the element C contained in the exhaustgas-purifying catalyst 1 is, for example, 7 atomic % or more, typically20 atomic % or more, more typically 30 atomic % or more, and still moretypically 35 atomic % or more. The ratio is, for example, 99 atomic % orless, and typically 95 atomic % or less.

The numerical values mentioned for the element C herein are valuesobtained for the exhaust gas-purifying catalyst 1 immediately afterbeing heated to high temperatures in an oxidizing atmosphere.

The catalytic metal 32 contains one or more precious metals. Theprecious metals are rhodium, palladium, platinum, or a combinationthereof. In the exhaust gas-purifying catalyst 1 immediately after beingheated to high temperatures in an oxidizing atmosphere, the preciousmetals exist as element metals for example. At this time, at least apart of the precious metals may be oxidized (the oxidation number isincreased). In the exhaust gas-purifying catalyst 1 immediately afterbeing heated to high temperatures in a reducing atmosphere, at least apart of the precious metals exist in the form of an alloy together withthe element C.

The ratio of the mass of the precious metals contained in the catalyticmetal 32 to the total of the mass of the support 31 and the mass of thecatalytic metal 32 is, for example, within a range of 0.01 to 10% bymass, and typically within a range of 0.1 to 5% by mass. When the massratio is decreased, the support 31 is required in a greater amount inorder to achieve high exhaust gas purification performance, which causesan increase in the thermal capacity of the exhaust gas-purifyingcatalyst 1. The increased mass ratio is apt to cause the sintering ofthe catalytic metal 32.

The ratio of the mass of the catalytic metal 32 to the volume of theexhaust gas-purifying catalyst 1 is, for example, within a range of 0.1to 20 g/L, and typically within a range of 1 to 10 g/L. The decreasedratio makes it difficult to achieve high exhaust gas-purificationperformance. The increased ratio causes an increase in cost of rawmaterials for the exhaust gas-purifying catalyst 1.

The numerical value mentioned for the catalytic metal 32 herein is avalue obtained for the exhaust gas-purifying catalyst 1 immediatelyafter being heated to high temperatures in an oxidizing atmosphere.

The catalyst layer 3 can further contain other components. For example,the catalyst layer 3 may further contain an oxygen storage material.

The oxygen storage material stores oxygen under an oxygen-rich conditionand emits oxygen under an oxygen-lean condition so as to optimize theoxidation reactions of HC and CO and the reductive reactions of NO_(x).The oxygen storage material is in the form of particles, for example.

The oxygen storage material is, for example, ceria, a composite oxide ofceria with another metal oxide, or a mixture thereof. As the compositeoxide, for example, a composite oxide of ceria and zirconia can be used.

The exhaust gas-purifying catalyst 1 provides a state change to bedescribed later under high temperature conditions.

FIG. 3 shows a state of the catalyst layer 3 of the exhaustgas-purifying catalyst 1 when being exposed to an atmosphere with a highoxygen concentration under high temperature conditions, for example,when the fuel supply to an engine is cut off. On the other hand, FIG. 4shows a state of the catalyst layer 3 of the exhaust gas-purifyingcatalyst 1 when being exposed to an atmosphere with a low oxygenconcentration under high temperature conditions, for example, when anabundance of fuel is continuously supplied to an engine, for example.Herein, the precious metal contained in the catalytic metal 32 is statedas palladium, and the element C is stated as iridium in order tosimplify the description thereof. In FIG. 3, reference character 31 arepresents iridium solid-solutioned in the support 31. In FIG. 4,reference character 32 a represents an alloy containing palladium andiridium.

In the state shown in FIG. 3, palladium contained in the catalytic metal32 exists as a simple metal, for example. In this state, at least a partof palladium may be oxidized.

When the oxygen concentration of the atmosphere under high temperatureconditions is decreased, the catalyst layer 3 of the exhaustgas-purifying catalyst 1 changes from the state shown in FIG. 3 to thestate shown in FIG. 4. Specifically, iridium is precipitated from thesupport 31, and at least a part of the precipitated iridium andpalladium form an alloy.

When the oxygen concentration of the atmosphere under high temperatureconditions is increased again, the catalyst layer 3 of the exhaustgas-purifying catalyst 1 changes from the state shown in FIG. 4 to thestate shown in FIG. 3. Specifically, at least a part of iridium formingan alloy together with palladium is solid-solutioned in the support 31,and palladium changes to a simple metal or its oxide from the alloy. Atleast a part of iridium existing as a simple metal is alsosolid-solutioned in the support 31.

Thus, the catalyst layer 3 of the exhaust gas-purifying catalyst 1reversibly changes with a change in the oxygen concentration of theatmosphere under high temperature conditions.

The melting point of the alloy containing iridium and palladium ishigher than the melting point of palladium. Therefore, when theatmosphere is at high temperatures and the oxygen concentration thereofis low, the sintering of palladium is less likely to occur.

When the atmosphere is at high temperatures and the oxygen concentrationthereof is high, iridium may volatilize as iridium oxide. However, whenthe oxygen concentration of the atmosphere under high temperatureconditions is increased, iridium is solid-solutioned in the support 31.The composite oxide containing iridium is less likely to volatilize ascompared with iridium oxide. Therefore, the exhaust gas-purifyingcatalyst 1 is less likely to cause the volatilization of iridium.

As apparent from the above description, the sintering of the preciousmetals contained in the catalytic metal 32 can be suppressed for a longperiod of time by adopting the above-mentioned configuration. That is,the exhaust gas-purifying catalyst 1 is less likely to causedeterioration in performance resulting from the sintering of theprecious metals.

The exhaust gas-purifying catalyst 1 is manufactured by the followingmethod, for example.

First, a support 31 is prepared. The support 31 is prepared by thefollowing method, for example. That is, coprecipitation is produced inan aqueous solution containing a salt of an element A and a salt of anelement B. Then, the aqueous solution is stirred, for example, at atemperature of 50 to 80° C. for 60 to 180 minutes, and an aqueoussolution containing a salt of an element C is then added to the stirredaqueous solution to further produce coprecipitation in the mixedsolution. Then, the coprecipitate thus obtained is dried, and fired inan oxidizing atmosphere. A firing temperature is set to be within arange of 500 to 1000° C., for example. As above, the support 31 isobtained.

Next, the support 31 is dispersed in deionized water to produce adispersion liquid. Subsequently, a solution containing a salt of atleast one precious metal of rhodium, palladium, and platinum is added tothe dispersion liquid, to cause the support 31 to adsorb the preciousmetal. Then, this is dried, and fired in an oxidizing atmosphere. Afiring temperature is set to be within a range of about 250 to about500° C., for example. As above, a supported catalyst containing thesupport 31 and a catalytic metal 32 supported by the support 31 isobtained.

Then, a slurry containing the supported catalyst is prepared. Anothercomponent, for example, an oxygen storage material is added to theslurry if necessary. Then, the slurry is coated on the substrate 2 toproduce a coated film, and the coated film is dried and fired. As above,an exhaust gas-purifying catalyst 1 is completed.

In the exhaust gas-purifying catalyst 1 described herein, the catalystlayer 3 has a single layer structure, but the catalyst layer 3 may havea multilayer structure. The monolith catalyst has been described herein,but the above-mentioned technique can also be applied to a pelletcatalyst.

Examples

Hereinafter, examples of the present invention will be described.

<Manufacture of Catalyst C1>

An exhaust gas-purifying catalyst was manufactured by the followingmethod.

First, a lanthanum nitrate aqueous solution containing 0.1 mol oflanthanum and an iron nitrate aqueous solution containing 0.099 mol ofiron were added to 500 mL of deionized water to produce a mixedsolution, and the mixed solution was stirred. Then, an aqueous solutioncontaining potassium hydroxide at the concentration of 20% by mass wasadded to the mixed solution until the pH of the mixed solution reached10 at room temperature so as to cause coprecipitation.

The aqueous solution was stirred at 70° C. for 150 minutes, and aniridium nitrate aqueous solution containing 0.001 mol of iridium wasthen added to the stirred aqueous solution to produce a mixed aqueoussolution. Then, an aqueous solution containing potassium hydroxide atthe concentration of 20% by mass was added to the mixed aqueous solutionuntil the pH of the mixed aqueous solution reached 12 at roomtemperature so as to cause coprecipitation.

Then, the solution was filtrated to produce a filter cake, and thefilter cake was washed with pure water. Subsequently, this was dried at110° C., and fired in the atmosphere at 1000° C. for 1 hour. As above, apowdery support was obtained.

A part of the powder was taken, and immersed for 12 hours in 25%hydrochloric acid held at room temperature. Note that this conditionallowed only the composite oxide of the above powder to be dissolved.Subsequently, the solution was filtrated to a produce a filtrate, andthe filtrate was subjected to inductively coupled plasma (ICP)spectrometry. As a result, the iridium content of the filtrate revealedthat 94% of iridium formed the solid solution, that is, the solidsolution-forming ratio was 94%.

The crystal structure of the powder was investigated according to X-raydiffraction. As a result, the powder was confirmed to have a perovskitestructure.

Next, the powdery support obtained by the above-mentioned method wasadded to 500 mL of deionized water. After the support was well dispersedin the deionized water by 10 minutes of ultrasonic agitation, apalladium nitrate aqueous solution was added to the slurry. Theconcentration and amount of the palladium nitrate aqueous solution wereadjusted such that a palladium ratio in a supported catalyst to beprepared was 0.5% by mass.

Then, the slurry was filtrated under suction to produce a filtrate. Thefiltrate was subjected to ICP spectrometry. As a result, it was revealedthat the filter cake contained almost the entire palladium in theslurry.

Next, the filter cake was dried at 110° C. for 12 hours. Subsequently,this was fired in the atmosphere at 500° C. for 1 hour. Thus, thepalladium was supported by the support.

The supported catalyst was then compression-molded to produce a moldedproduct, and the molded product was pulverized into pellets having aparticle diameter of 0.5 mm to 1.0 mm. As above, a pellet catalyst wasobtained as an exhaust gas-purifying catalyst. Hereinafter, the pelletcatalyst is referred to as a “catalyst C1”.

<Manufacture of Catalyst C2>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that firing for obtaining a support wasperformed at 950° C. instead of 1000° C. Hereinafter, the pelletcatalyst is referred to as a “catalyst C2”.

In the manufacture of the catalyst C2, the solid solution-forming ratioof iridium was measured by the same method as that performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 82%.

<Manufacture of Catalyst C3>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that firing for obtaining a support wasperformed at 900° C. instead of 1000° C. Hereinafter, the pelletcatalyst is referred to as a “catalyst C3”.

In the manufacture of the catalyst C3, the solid solution-forming ratioof iridium was measured by the same method as that performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 64%.

<Manufacture of Catalyst C4>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that firing for obtaining a support wasperformed at 800° C. instead of 1000° C. Hereinafter, the pelletcatalyst is referred to as a “catalyst C4”.

In the manufacture of the catalyst C4, the solid solution-forming ratioof iridium was measured by the same method as that performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 48%.

<Manufacture of Catalyst C5>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that firing for obtaining a support wasperformed at 700° C. instead of 1000° C. Hereinafter, the pelletcatalyst is referred to as a “catalyst C5”.

In the manufacture of the catalyst C5, the solid solution-forming ratioof iridium was measured by the same method as that performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 33%.

<Manufacture of Catalyst C6>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that firing for obtaining a support wasperformed at 600° C. instead of 1000° C. Hereinafter, the pelletcatalyst is referred to as a “catalyst C6”.

In the manufacture of the catalyst C6, the solid solution-forming ratioof iridium was measured by the same method as that performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 20%.

<Manufacture of Catalyst C7>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that firing for obtaining a support wasperformed at 500° C. instead of 1000° C. Hereinafter, the pelletcatalyst is referred to as a “catalyst C7”.

In the manufacture of the catalyst C7, the solid solution-forming ratioof iridium was measured by the same method as that performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 9%.

<Manufacture of Catalyst C8>

An exhaust gas-purifying catalyst was manufactured by the followingmethod.

First, a lanthanum nitrate aqueous solution containing 0.1 mol oflanthanum and an iron nitrate aqueous solution containing 0.1 mol ofiron were added to 500 mL of deionized water to produce a mixedsolution, and the mixed solution was stirred. Then, an aqueous solutioncontaining potassium hydroxide at the concentration of 20% by mass wasadded to the mixed solution until the pH of the mixed solution reached10 at room temperature so as to cause coprecipitation.

Then, the solution was filtrated to produce a filter cake, and thefilter cake was washed with pure water. Subsequently, this was dried at110° C., and fired in the atmosphere at 1000° C. for 1 hour. As above, apowdery support was obtained.

Next, the powdery support obtained by the above-mentioned method wasadded to 500 mL of deionized water. After the support was well dispersedin the deionized water by 10 minutes of ultrasonic agitation, apalladium nitrate aqueous solution was added to the slurry. Theconcentration and amount of the palladium nitrate aqueous solution wereadjusted such that a palladium ratio in a supported catalyst to beprepared was 0.5% by mass.

Then, the slurry was filtrated under suction to produce a filtrate. Thefiltrate was subjected to ICP spectrometry. As a result, it was revealedthat the filter cake contained almost the entire palladium in theslurry.

Next, the filter cake was dried at 110° C. for 12 hours. Subsequently,this was fired in the atmosphere at 500° C. for 1 hour. Thus, thepalladium was supported by the support.

The supported catalyst was then compression-molded to produce a moldedproduct, and the molded product was pulverized into pellets having aparticle diameter of 0.5 mm to 1.0 mm. As above, a pellet catalyst wasobtained as an exhaust gas-purifying catalyst. Hereinafter, the pelletcatalyst is referred to as a “catalyst C8”.

<Manufacture of Catalyst C9>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that an iron nitrate aqueous solutioncontaining 0.09995 mol of iron and an iridium nitrate aqueous solutioncontaining 0.00005 mol of iridium were respectively used instead of aniron nitrate aqueous solution containing 0.099 mol of iron and aniridium nitrate aqueous solution containing 0.001 mol of iridium.Hereinafter, the pellet catalyst is referred to as a “catalyst C9”.

Also in the manufacture of the catalyst C9, the solid solution-formingratio of iridium was measured by the same method as that performed inthe manufacture of the catalyst C1. As a result, the solidsolution-forming ratio was 95%.

<Manufacture of Catalyst C10>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that an iron nitrate aqueous solutioncontaining 0.0999 mol of iron and an iridium nitrate aqueous solutioncontaining 0.0001 mol of iridium were respectively used instead of aniron nitrate aqueous solution containing 0.099 mol of iron and aniridium nitrate aqueous solution containing 0.001 mol of iridium.Hereinafter, the pellet catalyst is referred to as a “catalyst C10”.

In the manufacture of the catalyst C10, the solid solution-forming ratioof iridium was measured by the same method as that performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 94%.

<Manufacture of Catalyst C11>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that an iron nitrate aqueous solutioncontaining 0.09 mol of iron and an iridium nitrate aqueous solutioncontaining 0.01 mol of iridium were respectively used instead of an ironnitrate aqueous solution containing 0.099 mol of iron and an iridiumnitrate aqueous solution containing 0.001 mol of iridium. Hereinafter,the pellet catalyst is referred to as a “catalyst C11”.

In the manufacture of the catalyst C11, the solid solution-forming ratioof iridium was measured by the same method as that performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 83%.

<Manufacture of Catalyst C12>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that an iron nitrate aqueous solutioncontaining 0.07 mol of iron and an iridium nitrate aqueous solutioncontaining 0.03 mol of iridium were respectively used instead of an ironnitrate aqueous solution containing 0.099 mol of iron and an iridiumnitrate aqueous solution containing 0.001 mol of iridium. Hereinafter,the pellet catalyst is referred to as a “catalyst C12”.

Also in the manufacture of the catalyst C12, the solid solution-formingratio of iridium was measured by the same method as that performed inthe manufacture of the catalyst C1. As a result, the solidsolution-forming ratio was 67%.

<Manufacture of Catalyst C13>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that a mixed solution of a lanthanumnitrate aqueous solution containing 0.09 mol of lanthanum and aneodymium nitrate aqueous solution containing 0.01 mol of neodymium wasused instead of a lanthanum nitrate aqueous solution containing 0.1 molof lanthanum. Hereinafter, the pellet catalyst is referred to as a“catalyst C13”.

Also in the manufacture of the catalyst C13, the solid solution-formingratio of iridium was measured by the same method as that performed inthe manufacture of the catalyst C1. As a result, the solidsolution-forming ratio was 91%.

<Manufacture of Catalyst C14>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that a mixed solution of a lanthanumnitrate aqueous solution containing 0.095 mol of lanthanum and anyttrium nitrate aqueous solution containing 0.005 mol of yttrium wasused instead of a lanthanum nitrate aqueous solution containing 0.1 molof lanthanum. Hereinafter, the pellet catalyst is referred to as a“catalyst C14”.

In the manufacture of the catalyst C14, the solid solution-forming ratioof iridium was measured by the same method as that performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 93%.

<Manufacture of Catalyst C15>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that a mixed solution of an ironnitrate aqueous solution containing 0.089 mol of iron and an aluminumnitrate aqueous solution containing 0.01 mol of aluminum was usedinstead of an iron nitrate aqueous solution containing 0.099 mol ofiron. Hereinafter, the pellet catalyst is referred to as a “catalystC15”.

In the manufacture of the catalyst C15, the solid solution-forming ratioof iridium was measured by the same method as that performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 90%.

<Manufacture of Catalyst C16>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that a ruthenium acetate aqueoussolution containing 0.001 mol of ruthenium was used instead of aniridium nitrate aqueous solution containing 0.001 mol of iridium.Hereinafter, the pellet catalyst is referred to as a “catalyst C16”.

In the manufacture of the catalyst C16, the solid solution-forming ratioof ruthenium was measured by the same method as that in the measurementof the solid solution-forming ratio of iridium performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 91%.

<Manufacture of Catalyst C17>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that a tantalum-n-butoxide aqueoussolution containing 0.001 mol of tantalum was used instead of an iridiumnitrate aqueous solution containing 0.001 mol of iridium. Hereinafter,the pellet catalyst is referred to as a “catalyst C17”.

In the manufacture of the catalyst C17, the solid solution-forming ratioof tantalum was measured by the same method as that in the measurementof the solid solution-forming ratio of iridium performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 79%.

<Manufacture of Catalyst C18>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that a pentaethoxy niobium aqueoussolution containing 0.001 mol of niobium was used instead of an iridiumnitrate aqueous solution containing 0.001 mol of iridium. Hereinafter,the pellet-catalyst is referred to as a “catalyst C18”.

In the manufacture of the catalyst C18, the solid solution-forming ratioof niobium was measured by the same method as that in the measurement ofthe solid solution-forming ratio of iridium performed in the manufactureof the catalyst C1. As a result, the solid solution-forming ratio was62%.

<Manufacture of Catalyst C19>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that a hexaammonium heptamolybdateaqueous solution containing 0.001 mol of molybdenum was used instead ofan iridium nitrate aqueous solution containing 0.001 mol of iridium.Hereinafter, the pellet catalyst is referred to as a “catalyst C19”.

In the manufacture of the catalyst C19, the solid solution-forming ratioof molybdenum was measured by the same method as that in the measurementof the solid solution-forming ratio of iridium performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 55%.

<Manufacture of Catalyst C20>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that an ammonium tungstate aqueoussolution containing 0.001 mol of tungsten was used instead of an iridiumnitrate aqueous solution containing 0.001 mol of iridium. Hereinafter,the pellet catalyst is referred to as a “catalyst C20”.

In the manufacture of the catalyst C20, the solid solution-forming ratioof tungsten was measured by the same method as that in the measurementof the solid solution-forming ratio of iridium performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 46%.

<Manufacture of Catalyst C21>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that a dinitrodiamine platinum nitrateaqueous solution containing 0.001 mol of platinum was used instead of aniridium nitrate aqueous solution containing 0.001 mol of iridium.Hereinafter, the pellet catalyst is referred to as a “catalyst C21”.

In the manufacture of the catalyst C21, the solid solution-forming ratioof platinum was measured by the same method as that in the measurementof the solid solution-forming ratio of iridium performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 16%.

<Manufacture of Catalyst C22>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that a dinitrodiamine platinum nitrateaqueous solution was used instead of a palladium nitrate aqueoussolution. Hereinafter, the pellet catalyst is referred to as a “catalystC22”.

In the manufacture of the catalyst C22, the solid solution-forming ratioof iridium was measured by the same method as that performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 92%.

<Manufacture of Catalyst C23>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that a rhodium nitrate aqueous solutionwas used instead of a palladium nitrate aqueous solution. Hereinafter,the pellet catalyst is referred to as a “catalyst C23”.

In the manufacture of the catalyst C23, the solid solution-forming ratioof iridium was measured by the same method as that performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 94%.

<Manufacture of Catalyst C24>

An exhaust gas-purifying catalyst was manufactured by the followingmethod.

First, commercially available γ-alumina powder was added to 500 mL ofdeionized water. The specific surface area of the alumina powder usedherein was 150 m²/g. After the alumina powder was well dispersed in thedeionized water by 10 minutes of ultrasonic agitation, a palladiumnitrate aqueous solution was added to the slurry. The concentration andamount of the palladium nitrate aqueous solution were adjusted such thata palladium ratio in a supported catalyst to be prepared was 0.5% bymass.

Then, the slurry was filtrated under suction to produce a filtrate. Thefiltrate was subjected to ICP spectrometry. As a result, it was revealedthat the filter cake contained almost the entire palladium in theslurry.

Next, the filter cake was dried at 110° C. for 12 hours. Subsequently,this was fired in the atmosphere at 500° C. for 1 hour. Thus, thepalladium was supported by the alumina support.

The supported catalyst was then compression-molded to produce a moldedproduct, and the molded product was pulverized into pellets having aparticle diameter of 0.5 mm to 1.0 mm. As above, a pellet catalyst wasobtained as an exhaust gas-purifying catalyst. Hereinafter, the pelletcatalyst is referred to as a “catalyst C24”.

<Manufacture of Catalyst C25>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C24 except that a dinitrodiamine platinum nitrateaqueous solution was used instead of a palladium nitrate aqueoussolution. Hereinafter, the pellet catalyst is referred to as a “catalystC25”.

<Manufacture of Catalyst C26>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C24 except that a rhodium nitrate aqueoussolution was used instead of a palladium nitrate aqueous solution.Hereinafter, the pellet catalyst is referred to as a “catalyst C26”.

<Manufacture of Catalyst C27>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that stirring at 70° C. for 150 minuteswas omitted in the preparation of a support. Hereinafter, the pelletcatalyst is referred to as a “catalyst C27”.

In the manufacture of the catalyst C27, the solid solution-forming ratioof iridium was measured by the same method as that performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 7%.

<Manufacture of Catalyst C28>

A pellet catalyst was manufactured by the same method as that describedabove for the catalyst C1 except that a neodymium nitrate aqueoussolution containing 0.1 mol of neodymium was used instead of a lanthanumnitrate aqueous solution containing 0.1 mol of lanthanum. Hereinafter,the pellet catalyst is referred to as a “catalyst C28”.

In the manufacture of the catalyst C28, the solid solution-forming ratioof iridium was measured by the same method as that performed in themanufacture of the catalyst C1. As a result, the solid solution-formingratio was 82%.

<Evaluation>

The endurance of the catalysts C1 to C28 was tested by the followingmethod.

First, each of the catalysts C1 to C28 was set in a flow-type endurancetest apparatus, and a gas containing nitrogen as a main component wasmade to flow through a catalyst bed at the flow rate of 500 mL/minutefor 30 hours. During this period, the temperature of the catalyst bedwas held at 1000° C. As the gas made to flow through the catalyst bed, alean gas and a rich gas were used, and these gases were switched atintervals of 5 minutes.

Note that the lean gas was a gas prepared by adding oxygen to nitrogenat the concentration of 3%, while the rich gas was a gas prepared byadding carbon monoxide to nitrogen at the concentration of 6%.

Then, each of the catalysts C1 to C28 was set in an atmospheric fixedbed flow reactor. Then, the temperature of the catalyst bed was raisedfrom 100° C. to 500° C. at the temperature increase rate of 12°C./minute and the exhaust gas-purifying ratio was continuously measuredwhile a model gas was made to flow through the catalyst bed. As themodel gas, a gas containing equivalent amounts of oxidizing components(oxygen and nitrogen oxides) and reducing components (carbon monoxide,hydrocarbons, and hydrogen), which were adjusted stoichiometrically, wasused. The results are shown in the following Tables 1 to 3.

TABLE 1 50% Support purification Catalytic metal Solid solution-temperature Amount forming ratio (° C.) Catalyst Element (mass %) Finalproduct composition Crystal structure (%) HC NOx C1 Pd 0.5LaFe_(0.99)Ir_(0.0094)O₃ Perovskite 94 367 365 C2 Pd 0.5LaFe_(0.99)Ir_(0.0082)O₃ Perovskite 82 371 368 C3 Pd 0.5LaFe_(0.99)Ir_(0.0064)O₃ Perovskite 64 377 373 C4 Pd 0.5LaFe_(0.99)Ir_(0.0048)O₃ Perovskite 48 384 380 C5 Pd 0.5LaFe_(0.99)Ir_(0.0033)O₃ Perovskite 33 392 388 C6 Pd 0.5LaFe_(0.99)Ir_(0.002)O₃ Perovskite 20 425 419 C7 Pd 0.5LaFe_(0.99)Ir_(0.0009)O₃ Perovskite  9 454 448 C8 Pd 0.5 LaFeO₃Perovskite — 464 461 C9 Pd 0.5 LaFe_(0.9995)Ir_(0.000475)O₃ Perovskite95 421 416 C10 Pd 0.5 LaFe_(0.999)Ir_(0.00094)O₃ Perovskite 94 380 377

TABLE 2 50% Support purification Catalytic metal Solid solution-temperature Amount forming ratio (° C.) Catalyst Element (mass %) Finalproduct composition Crystal structure (%) HC NOx C11 Pd 0.5LaFe_(0.9)Ir_(0.083)O₃ Perovskite 83 376 373 C12 Pd 0.5LaFe_(0.7)Ir_(0.201)O₃ Perovskite 67 415 411 C13 Pd 0.5La_(0.9)Nd_(0.1)Fe_(0.99)Ir_(0.0091)O₃ Perovskite 91 362 361 C14 Pd 0.5La_(0.95)Y_(0.05)Fe_(0.99)Ir_(0.0093)O₃ Perovskite 93 364 363 C15 Pd 0.5LaFe_(0.89)Al_(0.1)Ir_(0.009)O₃ Perovskite 90 362 360 C16 Pd 0.5LaFe_(0.99)Ru_(0.0091)O₃ Perovskite 91 371 366 C17 Pd 0.5LaFe_(0.99)Ta_(0.0079)O₃ Perovskite 79 384 380 C18 Pd 0.5LaFe_(0.99)Nb_(0.0062)O₃ Perovskite 62 391 385 C19 Pd 0.5LaFe_(0.99)Mo_(0.0055)O₃ Perovskite 55 400 397 C20 Pd 0.5LaFe_(0.99)W_(0.0046)O₃ Perovskite 46 417 413

TABLE 3 50% Support purification Catalytic metal Solid solution-temperature Amount forming ratio (° C.) Catalyst Element (mass %) Finalproduct composition Crystal structure (%) HC NOx C21 Pd 0.5LaFe_(0.99)Pt_(0.0016)O₃ Perovskite 16 445 439 C22 Pt 0.5LaFe_(0.99)Ir_(0.0092)O₃ Perovskite 92 380 398 C23 Rh 0.5LaFe_(0.99)Ir_(0.0094)O₃ Perovskite 94 321 310 C24 Pd 0.5 Al₂O₃γ-alumina — 471 467 C25 Pt 0.5 Al₂O₃ γ-alumina — 480 495 C26 Rh 0.5Al₂O₃ γ-alumina — 433 426 C27 Pd 0.5 LaFe_(0.99)Ir_(0.0007)O₃ Perovskite 7 457 450 C28 Pd 0.5 NdFe_(0.99)Ir_(0.0082)O₃ Perovskite 82 373 369

In Tables 1 to 3, the “final product composition” of the “support” is acomposition obtained from the atomic ratios of elements specified byelemental analysis, and a crystal structure specified using X-raydiffraction. Herein, oxygen vacancies are disregarded. Regarding theatomic ratios of the elements other than elements that can besolid-solutioned in a support such as Ir, since the values of addedamounts (charged amount) in the manufacture can be presumed to be equalto actual values, the values of the charged amounts were presumed as theactual values of the final product.

In Tables 1 to 3, the column denoted by “50% purification temperature”shows the lowest temperature of the catalyst bed at which 50% or more ofeach component contained in the model gas was purified. The columnsdenoted by “HC” and “NO_(x)” show the data for hydrocarbons and nitrogenoxides, respectively.

FIG. 5 is a graph showing an example of influence that the compositionof the support exerts on the performance of the exhaust gas-purifyingcatalyst. The data obtained for the catalysts C1 to C8 are shown in FIG.5. In FIG. 5, the abscissa represents the solid solution-forming ratioof iridium, and the ordinate represents the 50% purification temperaturefor NO_(x).

FIG. 6 is a graph showing another example of influence that thecomposition of the support exerts on the performance of the exhaustgas-purifying catalyst. The data obtained for the catalysts C1 and C8 toC12 are shown in FIG. 6. In FIG. 6, the abscissa represents β in thegeneral formula, and the ordinate represents the 50% purificationtemperature for NO_(x).

As shown in Tables 1 to 3 and FIG. 5, in the case where the compositeoxide contained in the support had a perovskite structure, and at leasta part of iridium was solid-solutioned in the support, higher exhaustgas-purification performance could be achieved as compared with the casewhere the composite oxide contained in the support did not have aperovskite structure or the case where iridium was not solid-solutionedin the support at all. In the case where the solid solution-formingratio of iridium was 20% or more, particularly excellent exhaustgas-purification performance could be achieved. In the case where thesolid solution-forming ratio of iridium was 30% or more, furtherexcellent exhaust gas-purification performance could be achieved.

As shown in Tables 1 to 3 and FIG. 6, in the case where β was more than0, higher exhaust gas-purification performance could be achieved ascompared with the case where β was 0. In the case where β was within arange of 0.000475 to 0.201, particularly excellent exhaustgas-purification performance could be achieved.

In the case where β was in a range of 0.00094 to 0.083, the mostexcellent exhaust gas-purification performance could be achieved.

The method of manufacturing the catalyst C27 is different from themethod for manufacturing the catalyst C1 only in that stirring at 70° C.for 150 minutes is omitted in the preparation of the support. However,the catalyst C27 has a remarkably lower solid solution-forming ratio ofiridium than that of the catalyst C1. This shows that a heat treatmentperformed immediately after a first coprecipitation process exerts largeinfluence on the solid solution-forming ratio of iridium.

When the data obtained for the catalysts C1 and C16 to C20 are compared,the catalysts C1 and C16 achieve more excellent performance than thoseof the catalysts C17 to C20. One of the reasons is considered to be thatiridium and ruthenium achieve a higher solid solution-forming ratio thanthose of tantalum, niobium, molybdenum, and tungsten, and are thus lessprone to cause evaporation. Another reason is considered to be thatiridium and ruthenium are likely to form an alloy together with rhodium,palladium, and platinum as compared with tantalum, niobium, molybdenum,and tungsten, and have a large effect of suppressing the sintering ofthe alloys.

The catalyst C21 could not exhibit catalyst performance that iscomparable to those of the catalysts C1 and C16 to C20. This resultshows that platinum is not suitable as an element solid-solutioned in asupport such as iridium.

<Analysis>

Each of the catalysts C1 and C24 was set in a flow-type endurance testapparatus, and a gas containing nitrogen as a main component was made toflow through a catalyst bed at the flow rate of 500 mL/minute for 30hours. During this period, the temperature of the catalyst bed was heldat 1000° C. As the gas made to flow through the catalyst bed, a lean gasand a rich gas were used, and these gases were switched at intervals of5 minutes.

Note that the lean gas was a gas prepared by adding oxygen to nitrogenat the concentration of 3%, while the rich gas was a gas prepared byadding carbon monoxide to nitrogen at the concentration of 6%. Theendurance test was performed such that the rich gas was finally made toflow through the catalyst bed.

After the endurance test, the catalyst layer of each of the catalysts C1and C24 was observed by FE-SEM (field emission-scanning electronmicroscope).

FIG. 7 is an electron microscope photograph of the catalyst layerobtained for the catalyst C1 after the endurance test. FIG. 8 is anelectron microscope photograph of the catalyst layer obtained for thecatalyst C24 after the endurance test.

As shown in FIG. 8, in the catalyst layer of the catalyst C24, theaverage particle diameter of palladium was about 60 nm. In contrast, inthe catalyst layer of the catalyst C1, as shown in FIG. 7, the averageparticle diameter of palladium was about 30 nm.

The catalyst layer of the catalyst C1 was subjected to EDX (fieldemission-scanning electron microscope-energy dispersive X-ray) analysis.As a result, the catalytic metal was confirmed to be made of an alloycontaining palladium and iridium at the atomic ratio of 8:2.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope and,spirit of the inventions.

What is claimed is:
 1. An exhaust gas-purifying catalyst comprising: asupport including a composite oxide having a composition represented bya general formula AB_(α)C_(β)O₃, wherein A represents one or moreelements selected from the group consisting of lanthanum, neodymium, andyttrium, B represents iron or a combination of iron and aluminum, Crepresents one or more elements selected from the group consisting ofiridium, ruthenium, tantalum, niobium, molybdenum, and tungsten, α and βeach represents a numerical value within a range of more than 0 and lessthan 1, and α and β satisfy relational formulae of β<α and α+β≦1; and acatalytic metal as one or more precious metals supported by the support.2. The exhaust gas-purifying catalyst according to claim 1, wherein aproportion of an amount of the element C contained in the compositeoxide in a total amount of the element C contained in the exhaustgas-purifying catalyst is 20 atomic % or more.
 3. The exhaustgas-purifying catalyst according to claim 1, wherein α is within a rangeof 0.7 to 0.9995, and β is within a range of 0.000475 to 0.201.
 4. Theexhaust gas-purifying catalyst according to claim 1, wherein thecomposite oxide contains a compound having a perovskite structure. 5.The exhaust gas-purifying catalyst according to claim 1, wherein thecatalytic metal contains palladium.